University of Guelph paper- Flushing is a myth!

StonedPone said:

What do you think?

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Flushing is a waste of time, imnsho.

The idea first started with hydro growers back in the early to mid '90s, was total and complete bullshit then... as it is now.

If you have a basic understanding of botany, you know why the basic concept is fatally flawed.

But just like cutting back fertilizer close to harvest to induce a "fade", old bro science ideas never fucking die.

A sap test would be interesting. That might as simple as having a chew on some twig.

I feel there can be a difference, but you make that difference in a day. It's simply about what is in the moisture content, as you chop. All the P fixed in during bloom isn't washing out. The feed used has left it's mark.

I started to put less importance on flushing, long before you could admit to such things. As a hydro guy, I had no build-ups to shift out my compost, so saw no logical need for days of flush. It just gives dying plants. However that one day, seemed to make a difference. Which is bending the idea of what a flush is really. Something we have no real definition of anyway. I just think I can tell if the plants were fed that day, or not, in hydro. Hence, I should revisit this with some stem chewing. Not just gargling tank water.

Overall I see no gain, and I'm happy to see the forum is also leaning that way. Time is money, so time spent flushing should bring a gain that pays for that time. If we take a 7 week plant to 8 weeks with flushing, I need to see a 15% gain, to break even. I don't need leaf drop, or throttled growth. That's not a happily performing plant. It would be a bad week, before chop.


A point worth airing, is that most P in our plant is accumulated at luxury levels, just sat around, ready for seed production. This means that withholding P at the end, isn't making the plant use up what it has. As if there is some process that will make the P into something more palatable. That P is just sat there. It's going nowhere.

The one time I flushed my plants was due to salt buildup...otherwise I leave it to the toilet...

StonedPone said:

Great conversation!
Let's assume that flushing is pointless, yet a number of people claim to get some weird flavors or a harshness typically not associated with weed.
Crusty suggests carbon being the source, I find this plausible, hear me out.
As a homebewer I keg my beers and apply co2, if I get it wrong I get a carbonic acid burn when tasting the beer. ( think the burn when drinking Coke)
So could what people are getting is carbonic acid ? Someone in another thread mentioned that he was curing his jars upside down on a screen so as to allow the co2 escape.
Without a way for the co2 to escape in a closed jar, I would think it would have a negative effect on the weed.
What do you think?

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Flushing makes no difference at all. Complete bro science.

JMO but I think you're better off imitating nature and dealing with the rest of the variables best you can like lowering the temp at night light distance/intensity etc and adding other pressures now and then , as there will still be fertilizer in the ground when they ripen outside. I've never flushed and I don't believe one can add more ferts during flowering to make bigger buds and force feed either,they are somewhat selective and as long as there is enough and they don't run out,that's all you need. Others may know more but it works for me

How do you explain bud that burns black and sooty or sizzles and sparks, that is harsh and tastes like crap?

Of course you can't literally rinse the plant out by pouring huge volumes of water through the medium, but plants uptake water and if there are more nutrients in the water than they can metabolize where does it end up, do they just magically uptake the water and leave the nutrients behind?

mexweed said:

How do you explain bud that burns black and sooty or sizzles and sparks, that is harsh and tastes like crap?

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Possibly use of PGR's but most likely poor harvesting, drying and curing technique.

You can actually water cure your bud if that's the case, not the best smoke, but worth it when dealing with shwag

here’s some statements I’ve found that were liked by jbeezy when he posted about this topic. The order is scrambled btw

One thing I’m trying to hammer down is getting the super chalk white ash and not the usual salt with a slight dash of pepper

Anyone care to chime in on how to get it there in no till?

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sulfates help ripen. and the plant isn't dying, you're just coercing it to a slow senescence through a proper taper of input bulk EC, environmental cues, light intensity, CO2, (high light+co2 at the end of the cycle = stored sugar in flowers that will not combust). calcium sulfate seems like a good idea for crop longevity once harvested and for translocation and metabolization of sugars and complex molecules. but only when combined with the factors above.



so keep the lights & co2 high all the way to the end? And any recommendations on a quality sulfate product?



it's a gradual taper, and then ambient CO2 levels for the final 10-12 days ish.. depending on how the crop ripens up. you just gotta think. the process of photosynthesis is literally turning light and carbon dioxide into sugars. sugars don't tend to combust very well, leaving a black ash and a caramelized taste.



so if I lower co2 levels then I would have to lower light levels also to avoid bleaching right? Being that led's are inches away from canopy ?



they play together. high light intensity requires that CO2 for photosynthesis. so they have to taper together. i can't give you exact specifics but i would play with tapering levels in your own rooms. there are a lot of factors at play, and there is no single answer. you must fundamentally understand what's happening in & around the plant based on your inputs.



Some people think if you totally stop feeding all elements in Coco and rock wool the plant will hold onto nutrients instead of consuming them. Same friend says to decrease Cal Nitrate and replace with Calcium sulfate.



Finished & flushed are two different things.

Most people's problems come from too much

Magnesium at the end. Calcium is for finishing, not mag



Flushing translocates phosphorus, k and other micros from the rootball to the plant tissue. Opposite. Dialing down cal nitrate towards the end and replace with cal chloride.



Mineral content does not change feeding plain water vs feeding minerals. The point of flush is to force finish and bag appeal. Stop feeding nitrogen the final 2 weeks and give it what it needs to put out fire. We have been ahead of the game.



Less nitrogen less magnesium no flush needed



it's about a calculated wind down of nutrients so the plants naturally senesce and use up what they have stored in their tissue and root zone. Just waiting until the last 7 days and then blasting them with straight water, using Fade or Winterfrost is not actually helping that process. It's masking it and just shutting them down



ash color has damn near nothing to do with moisture content. Damn near nothing to do with the dry. And everything to do with how the tissue is built. Now moisture will impact how smooth it is (lil wet it will be a lil hot). Heavy Calmag offers fake white ash that looks like plywood and burns better than it would have potentially. But by no means does heavy Calmag do it, otherwise everyone would have good smoking flower.

And last time I checked, that's far from the case.



I did notil for 3 years.

You have to let the plant finish its cycle and it can't really be manipulated to finish faster in notil, so if it didn't break everything down it's smoking peppered and personal I don't like it. I personally think tapering down closer to harvest when using salts and then flushing is a must. Notil gotta let the plant finish. Dumping water in isn't going to help notil much.

Never been an issue for me, but in both cases the bud was dried and cured properly, the black and sooty was from a buddy's soil grow, the sparking has happened a few times from other buddies' grows, the pgr theory is interesting because foliar spraying with kelp stuff is pretty common and there is something in kelp that is technically a pgr

Everyone else that was smoking it thought it was dank, but it tore up my throat, it actually tasted like the flavor of the strain but way over saturated not in a good way, at the time I thought it had probably been loaded up with bud candy or something up until chop

mexweed said:

How do you explain bud that burns black and sooty or sizzles and sparks, that is harsh and tastes like crap?

Of course you can't literally rinse the plant out by pouring huge volumes of water through the medium, but plants uptake water and if there are more nutrients in the water than they can metabolize where does it end up, do they just magically uptake the water and leave the nutrients behind?

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Sugar! and bugs! tiny mites, etc ( and yes your indoor buds have them too ).
Also this "popping and sparking" has been attributed to Magnesium. So show me 50 ppms of Mag set on fire......

So when you flush a plant, what happens? The final act of any plant is preservation of it's fertilized flowers. So WHERE does the nutrients in the plant go as it is starving to death? The buds! so all the stuff you think your getting rid of is merely migrated to the buds! Unless you starve it until the buds die also! Who wants dead, brown buds.

The Mg won't flash without a decent oxidiser. That's the K. The heat from a high K burn leaves the whitest ash. You don't flush for a white ash, you load with K for the most complete burn. People chasing the white ash tend to be convinced low feed is the answer. They may even be convinced enough, to pretend they have achieved it. It's K though, and you can apply it during curing if you wish.

Some things are really better left unsaid though..

Ca++ said:

You don't flush for a white ash, you load with K for the most complete burn.

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That's an interesting take, completely the opposite of what old tobacco references say. Tobacco is said to be best with high sugars content too.

I'd think that starch and cellulose are a problem, rapid drying creating a worse product and all.

DoublePlatinum said:

a calculated wind down of nutrients so the plants naturally senesce

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In my hydro hands it's best to stay between 500-700 ppm for the whole grow - and not having test results to consult, I suspect that many harvest later than they should, not only losing THC but increasing the risk of bud rots, which I've never seen in a plant actively taking up water.

G.O. Joe said:

That's an interesting take, completely the opposite of what old tobacco references say. Tobacco is said to be best with high sugars content too.

I'd think that starch and cellulose are a problem, rapid drying creating a worse product and all.

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Here's a tobacco grower saying how he does it, and offering a paper. Though he doesn't realise it's an oxidiser. Which is why it's found in black powders.
I couldn't find an actual brix for tobacco when I looked. I did see high sugar doesn't burn well though. As you might expect, it you have ever burnt sugar. People had bad things to say about high and low effects, but without some numbers to quantify this, it's a bit mute. We don't crop during peak lights on sugar production though.
They seem very interested in product pH. I didn't read about it though. It's probably transferable, as they will adjust that during the cure also. It's not a plant health things, it's smoking quality.

Ca++ said:

Here's a tobacco grower saying how he does it, and offering a paper. Though he doesn't realise it's an oxidiser. Which is why it's found in black powders.

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Yes well he's talking about spraying dead tobacco with K carbonate which is a rather different thing than fertilization - it's a strongish base as said in the thread and is undoubtedly causing chemical changes which you're not going to get in any other way. It's not the K that's the oxidizer in gunpowder, it's the nitrate of course. Excess nitrate should definitely help ganj burn and the plant is a sponge for it, though it probably has downsides.

If you're going to spray something on your gear, enzyme isolates from various species (combinations are better than any pure strain) of bacteria is the shit: https://doi.org/10.3389/fmicb.2023.1211936

This is where the spraying of cured tobacco with K2CO3 solution in the tobacco forum thread comes from:


Many things are tested and the author does not reach any conclusions as to the why of "The alkali carbonates of caesium, rubidium, and potassium have a definite marked effect in promoting the fire-holding capacity of tobacco"

It cites earlier work such as "The relation of the composition of the leaf to the burning qualities of tobacco" which can be found at
and has a few interesting bits to it. It says that "fire-holding capacity" of tobacco is extractable and transferable, and notes that the K content is extractable and Ca is not. This is with water - alcohol extraction of tannin, sugars, etc does not improve the residue. It says that this same "fire-holding capacity" is not quite the droids we're looking for: "Although the organic potassium salts greatly favor the fire-holding capacity, they tend to produce a mottled, dark-colored ash." It goes on to say "tobacco containing excessive amounts of lime gives an ash which, although it is very light in color, lacks cohesion, or, in the language of the trade, it “flakes.” This is a very objectionable property and must always be taken into account in judging the burning qualities of tobacco. The potash salts, more especially the organic compounds, yield an ash which is firm and compact but dark in color. From these facts, then, it is clear that potash and lime combined in the proper proportion are essential to a firm, light-colored ash."

To really confuse things, "it frequently happens that samples very rich in potash have a poor burn, while others comparatively poor in this constituent show excellent burning qualities."

It doesn't have to be potassium nitrate in powders. Nitrogen is a very good oxidiser, but have you seen K put in water. It's the explosive one from grade school. It can form a number of compounds that aid burning. The authors lack of knowledge shows in the poster, but it's not really their topic.
They sprayed it on, but this thread stems from us all seeing it sparkle, fed at high rates. Neither is an exclusive method of getting it in there.

That thread was just what I found today, as you referenced tobacco.

People that have had spliffs like sparklers (which often use potassium chlorate) may recall the ash was white. Which would be useful input here. Though there is no guaranty that smoking a sparkler led to white ash, it's quite likely.

Ca++ said:

It doesn't have to be potassium nitrate in powders. Nitrogen is a very good oxidiser, but have you seen K put in water. It's the explosive one from grade school.

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It apparently does have to be KNO3 in black powder - and not the Na salt, for reasons which I don't know but could probably find. It's the sulfur that's optional. Oxygen oxidizes. Potassium (or sodium, rubidium, and cesium) explodes in water because it reduces water, which is the oxidizer: 2 K + 2 H2O = 2 KOH + H2, very exothermically, and the hydrogen then explodes in air: 2 H2 + O2 = 2 H2O

[NaNO3 with little to no KNO3 can indeed be used in black powder - it's more difficult to ignite and burns slower, but is more powerful]

Yeah... adding stuff to make your excess nutes burn better (hotter) is not the path to superior quality.

The key is to avoid the excesses in the first place.

CLEAN, non-overfed cannabis smoke is so cool in temp it can be difficult to 'feel' in your lungs. The smoke should NEVER be hot.